Use --fast by default in build_image when --jobs is enabled.

build_image

 #!/bin/bash
 
 # Copyright (c) 2009 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Script to build a bootable keyfob-based chromeos system image from within
 # a chromiumos setup. This assumes that all needed packages have been built into
 # the given target's root with binary packages turned on. This script will
 # build the Chrome OS image using only pre-built binary packages.
@@ skipping 74 matching lines @@
 if [ -z "${FLAGS_board}" ] ; then
   error "--board is required."
   exit 1
 fi
 
 if [ "${FLAGS_rootfs_size}" -gt "${FLAGS_rootfs_partition_size}" ] ; then
   error "rootfs (${FLAGS_rootfs_size} MB) is bigger than partition (${FLAGS_rootfs_partition_size} MB)."
   exit 1
 fi
 
+if [ ${FLAGS_jobs} -ne -1 ]; then
+  EMERGE_JOBS="--jobs=${FLAGS_jobs}"
+  FLAGS_fast=${FLAGS_TRUE}
+fi
+
 EMERGE_BOARD_CMD="emerge-${FLAGS_board}"
 if [ "${FLAGS_fast}" -eq "${FLAGS_TRUE}" ]; then
   echo "Using alternate emerge"
   EMERGE_BOARD_CMD="${SCRIPTS_DIR}/parallel_emerge --board=${FLAGS_board}"
 fi
 
 # Determine build version.
 . "${SCRIPTS_DIR}/chromeos_version.sh"
 
 # Use canonical path since some tools (e.g. mount) do not like symlinks.
@@ skipping 49 matching lines @@
 if [[ ${FLAGS_installmask} -eq ${FLAGS_TRUE} ]] ; then
   INSTALL_MASK="${DEFAULT_INSTALL_MASK}"
 fi
 
 # Reduce the size of factory install shim.
 # TODO: Build a separated ebuild for the factory install shim to reduce size.
 if [[ ${FLAGS_factory_install} -eq ${FLAGS_TRUE} ]] ; then
   INSTALL_MASK="${INSTALL_MASK} ${FACTORY_INSTALL_MASK}"
 fi
 
-if [[ ${FLAGS_jobs} -ne -1 ]]; then
-  EMERGE_JOBS="--jobs=${FLAGS_jobs}"
-fi
-
 # Figure out ARCH from the given toolchain.
 # TODO: Move to common.sh as a function after scripts are switched over.
 TC_ARCH=$(echo "${CHOST}" | awk -F'-' '{ print $1 }')
 case "${TC_ARCH}" in
   arm*)
     ARCH="arm"
     ;;
   *86)
     ARCH="x86"
     ;;
@@ skipping 516 matching lines @@
   echo "Developer image created as ${DEVELOPER_IMAGE_NAME}"
 fi
 
 print_time_elapsed
 
 echo "To copy to USB keyfob, OUTSIDE the chroot, do something like:"
 echo "  ./image_to_usb.sh --from=${OUTSIDE_OUTPUT_DIR} --to=/dev/sdX"
 echo "To convert to VMWare image, OUTSIDE the chroot, do something like:"
 echo "  ./image_to_vmware.sh --from=${OUTSIDE_OUTPUT_DIR}"
 echo "from the scripts directory where you entered the chroot."